The present invention relates to textured articles and in particular to the restoration of the microstructure of textured articles. The invention is further related to the field of refurbishment of gas turbine airfoils.
An example of the background technology of the present invention is U.S. Pat. No. 5,611,670 which shows a gas turbine blade made of a nickel based superalloy. Superalloys are nickel- or cobalt-based alloys, typically comprising chromium, titanium, tantalum, aluminum, tungsten and other elements, with excellent high temperature resistance, thereby maintaining high strength properties. Accordingly, superalloys are widely used in high temperature applications where additionally high mechanical strength is required. A typical application is the casting of airfoils for gas turbines, jet engines as well as stationary gas turbines, e.g. for industrial applications like power generation. Further improvements in mechanical strength is achieved by casting the superalloy as a columnar or as a single crystal. A textured article has no or very few grain boundaries.
As additional background, European Patent Application EP 1 038 982 A1 describes a process for manufacturing single crystal superalloy articles. After casting the article is subjected to a heat treatment in order to further improve the mechanical strength. The heat treatment is a high temperature solution heat treatment which homogenizes the microstructure of the alloy itself formed by different crystal phases. However, this heat treatment may lead to a grain re-crystallization occurrence, initiated by dislocations in the crystal structure. This grain re-crystallization destroys locally the single crystal structure which may lead to a dramatic decrease in the mechanical strength of the article. Accordingly, grain re-crystallization is a cause for rejection of single crystal castings if present beyond a preset maximum for re-crystallized grains and can result in low yields of acceptable heat-treated single crystal castings. By heat treating in a carburizing atmosphere, carbon is introduced into the casting and forms carbides therein that reduce or localize the occurrence of grain re-crystallization.
European Patent EP 0 525 545 B1 describes the refurbishment of corroded superalloy articles. In particular gas turbine airfoils are subjected to corrosion by hot gases. Typically, a corrosion protective coating is provided on the body made from the superalloy. Widely used coatings are of the MCrAlY type, where M is iron, cobalt and/or nickel, and Y, for yttrium or another rare earth element or another element such as lanthanum. This type of coating is usually applied by a plasma spray process. However, despite a corrosion protective coating, the airfoils are still under corrosion and erosion attack which leads to the need for servicing after a certain period of time. Corrosion results from contaminants in the fuel and/or air, and oxidation may also occur at high temperatures. Depending on the conditions of operation, an oxide layer of varying thickness may form on the surface of the airfoil. Also, and very significantly, sulfur can penetrate into the base material to form sulfides. Furthermore, internal oxides and nitrides may form within the metal near the surface.
Instead of completely exchanging airfoils, it is often a cost saving option to refurbish the airfoils, i.e. providing a new protective coating. This requires complete removal of the old coating, which is realized by applying mechanical stripping as well as chemical treatment, e.g. with acid. After removal of a substantial part of the old coating, the surface is aluminized. Subsequently, the aluminide layer is removed, thereby also removing oxidized and corroded regions at the surface.
U.S. Pat. No. 5,413,648 discloses a directionally solidified article with a plastic deformation damage at the surface, which is prone to recrystallization. This problem is overcome by removing a part of the deformed surface region.
European patent No. EP 1 036 850 A1 discloses a single crystal having a surface coating for preventing recrystallization fracture by reinforcing the grain boundaries. After a heat treatment the texture of the article at the surface shows no single crystal structure anymore, because the surface has grain boundaries, which are reinforced by grain boundary strengthening elements like Zr, Hf, B or C.
U.S. Pat. No. 6,271,668 shows the need of a coating during one step of refurbishment of gas turbine components, which is applied on the surface of the component. The coating is heat treated by which a surface region of the article is aluminized. This heat treatment is performed at low temperatures in order to avoid detrimental diffusion of atoms from corrosion products. The coating is removed together with the corroded layers before further heat treatments are performed.
The present invention provides a method for restoring the microstructure of a textured article, e.g. a single crystal or a directionally solidified article, which comprises creating on the surface of the article a high temperature stable surface coating and subsequently performing a solution heat treatment, thereby maintaining said thermally stable surface coating.
As stated previously, grain recrystallization may occur during a solution heat treatment of the article. The present invention underlies the discovery that grain recrystallization occurs at lower temperatures at the surface of an article compared to bulk regions. The energy needed for forming new grains with grain boundaries is lower at the surface. By applying a coating on the surface and maintaining this surface coating during the solution heat treatment, grain recrystallization is suppressed due to the now provided bulk conditions. Accordingly, an effective solution heat treatment can be processed, thereby restoring the microstructure of the textured article without introducing grain recrystallization. A full description of the effect of suppressing grain recrystallization by surface coating was given by one of the applicants in the publication xe2x80x9cRecrystallization In Single Crystals Of Nickel Base Superalloysxe2x80x9d, R. Bxc3xcrgel, P. D. Portella, J. Preuhs, Superalloys 2000, edited by T. M. Pollock, pages 229-238, the teaching of which is incorporated herein by reference, in particular with respect to the alloy compositions disclosed in table I (balance Ni) and heat treatment parameters in table II.
The article is preferably made from a superalloy, which may be nickel-based or cobalt-based. The microstructure in such a nickel-based superalloy is formed by a xcex3-phase and a xcex3xe2x80x2-phase. The temperature required for solution heat treatment is considerably high and is at least the solution temperature of the xcex3xe2x80x2-phase. By maintaining such a temperature during the solution heat treatment, an effective restoring of the microstructure is achieved. This high temperature normally increases the risk of recrystallization. This risk is substantially reduced by applying the surface coating before performing the solution heat treatment. The temperature of the solution heat treatment is preferably above 1100xc2x0 C., more preferred above 1150xc2x0 C. and even more preferred above 1200xc2x0 C.
Preferably, the article is a gas turbine blade or vane. The rejection rate of textured castings because of grain recrystallization is in particular high for gas turbine blades because very high mechanical stresses resulting from centrifugal forces require high mechanical strength which is strongly influenced and decreased by grain recrystallization. Moreover thermal fatigue cracking of blades or vanes may start from non-textured areas.
Preferably, the surface coating is an aluminide coating, which is preferably placed on the surface by a Chemical Vapor Deposition (CVD) process, which is well known in the art, e.g. for aluminizing articles for establishing corrosion resistance.
Some preferred composition of the surface coating with its main components are listed here (weight prozent):
Ni: 24%, Cr: 17%, Al: 45%,
Ni: 49%, Cr: 4%, Al: 37%,
Ni: 49%, Cr: 7%, Al: 35%.
The Al-content should be at least 2 wt % for an aluminide coating.
The surface coating could also be for instance an oxide film, e.g. developed by oxidation of the surface of the article, e.g.: alumina, NiO, chromoxide or mixtures of it. The oxidation of the article can be performed in air or in certain atmospheres with a given oxygen partial pressure in a pre-treatment leading to an oxide coating and then performing 3a solution heat treatment. The oxidation of the article can also be performed in one step during heat-up of the solution heat treatment. First an atmosphere suitable for oxidation is present and after oxidation has occurred a vacuum is applied in order not to oxidise the blade material too much.
Nevertheless the whole solution heat treatment can also be performed in a controlled oxidizing atmosphere. The oxidation of the surface is especially useful, if the article shows a plastic deformation of the surface, e.g. by strong grinding or other mechanical surface work such as shot peening. Nevertheless a thermally stable oxide layer can be applied to the surface by plasma spraying without oxidation of the surface of the article.
The invention is in particular useful for the refurbishment of a gas turbine blade. The invention provides a method for refurbishing a gas turbine blade made from a textured superalloy body coated with a protective coating and comprising the following subsequent steps: Coating the protective coating with a high temperature stable surface coating; performing a solution heat treatment of the superalloy, thereby maintaining the thermally stable surface coating; removing jointly the surface coating and the protective coating and finally providing a new protective coating on the body for the next engine operation period.
This refurbishment not only includes the re-coating of a protective coating system but also includes a solution heat treatment for re-establishing the full mechanical strength properties of the textured structure. Since the gas turbine blade was already subjected to erosion and corrosion attack, plastic deformations in the surface rim are likely to occur. As mentioned above, those plastic deformations are a source of grain recrystallization. Accordingly, in the past, the refurbishment did not include full solution heat treatment of the gas turbine blade because grain recrystallization would have occurred. The heat treatment had to be restricted to a lower temperature heat treatment for bonding the new protective coating. Restoring of the microstructure was not or not completely possible with such a treatment.
Now, a solution heat treatment can be performed without recrystallization because the surface coating or the oxide film also covers areas where the old protective coating is eroded and establishes bulk conditions for the gas turbine blade body which leads to a higher temperature threshold for grain recrystallization.
Depending on the actual condition of the old protective coating, the following method can be applied instead of the above-mentioned method. The following subsequent steps are performed, achieving the same advantages mentioned above: removing the protective coating; coating the surface of the gas turbine blade or vane with a high temperature stable surface coating; subsequently performing a solution heat treatment, thereby maintaining the thermally stable surface coating; removing the surface coating and providing a new protective coating on the body.
The invention is explained in greater detail below and by reference to exemplary embodiments shown in the drawings wherein like numerals refer to equivalent elements.